The current method of manufacturing scrolls is derived from a molten metal process (“casting”). Typically, the liquid gray cast iron is specially alloyed, inoculated, and poured into a cavity that then forms the scroll after solidification is complete. The current casting process produces a raw casting scroll with linear dimensional accuracy of about +/−0.020 inch per inch. Moreover, because of intrinsic metallurgical surface anomalies or defects caused by casting, extra machining stock (about 0.060 inch) must be added in addition to this tolerance resulting in about 0.060+/−0.020 inch total stock and variation to be machined off. The skin effect is produced because of the complicated thermodynamic, kinetic and metallurgical/chemical interactions that take place at the solidifying and cooling sand (or ceramic) to metal interface.
Molds used in the casting process, in which the molten metal flows into, are composed of sand, binder, and/or a ceramic coating and are not fully structurally rigid. When the liquid iron contacts the mold wall surfaces, pressure is exerted on the mold, which causes mold wall expansion. Gray cast iron is especially prone to solidification expansion because of the high carbon or graphite content. This phenomenon is a major source of dimensional variation and tolerance increases, as stated.
Scrolls, to perform properly, must not leak, wear out or fracture, so very accurate final dimensions must be held. To accomplish this, very extensive, complicated and expensive machining takes place on the raw castings to convert them into a useable scroll with the current casting manufacturing approach. Therefore, because of the aforementioned capability of the current casting processes, the excessive machining stock presents a major impediment to high volume productivity because of the shear amount of material needed to be machined off. The region of the scroll that is the most difficult to machine is the involute scroll form itself. Milling of this portion causes the most tool wear and takes the longest time to machine. The dimensional accuracy in the “involute scroll form” is, therefore, the most important.
The two fundamental types of powder metal manufacturing processes described herein enable the manufacturing of scroll with less “skin effect” layer and better dimensional tolerances while still meeting the rigorous stress and pressure requirements needed for a functioning scroll. They are metal injection molding and conventional press and sinter powder metallurgy. Both processes will have embodiments associated with them that will make the use of powder metallurgy practical and useful for manufacturing of near nets or net shaped scrolls. The scroll is either formed wholly or formed in parts and then joined to make the entire scroll component.
In general, the invention is directed towards the use of powder metals in the formation of a scroll component for a scroll compressor. It is envisioned that the entire scroll component can be formed utilizing powder metal techniques. It is further envisioned, that portions of the scroll compressor members can be produced utilizing powder metallurgy techniques. These portions such as the scroll's involute component, which requires an extremely high degree of dimensional tolerance, are then fastened to other portions of the scroll component which are formed by techniques such as casting, forging, or even another powdered metal part.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.